Evidence for widespread ∼1760 Ma metamorphism and rapid crustal stabilization of the early proterozoic (1870-1820 Ma) Penokean Orogen, Minnesota

Abstract

Regional amphibolite facies metamorphism of Early Proterozoic rocks in central Minnesota has historically been attributed solely to the 1870 to 1820 Ma Penokean collisional orogeny. New thermochronologic, thermobarometric, and geobarometric data suggest that in the internal zone of the orogen widespread staurolite-grade metamorphism occurred simultaneously with intrusion of post-Penokean (∼1770-1760 Ma) plutons. We propose a P-T-t path for Early Proterozoic evolution in which Penokean-age primary garnet growth (exhibited by inclusion-rich cores) occurred during tectonic burial to pressures of 5 to 6 kb. Inclusion-free garnet overgrowths and post-kinematic staurolite formed at similar pressures but well after the orogeny and during an episode of lower crustal fusion at 1770 to 1760 Ma. Discordance between garnet rim pressures and final pluton emplacement pressures (∼3.5 kb) indicates rapid uplift concomitant with magmatism and is supported by thermochronologic evidence for regional rapid cooling from above 500°C to below ∼300°C at 1750 to 1760 Ma. The proposed P-T-t path suggests that crust thickened during the Penokean orogeny remained thick for 50 to 60 my. Collapse of the overthickened orogen is characterized by sudden lower crustal melting, regional mid-crustal amphibolite facies metamorphism, and subsequent rapid uplift and cooling. Orogenic collapse appears to have been an important step in the stabilization of this part of the Proterozoic crust in the North American mid-continent Recognition and characterization of post-Penokean metamorphism in the North American midcontinent is essential for understanding post-orogenic cratonic evolution in the Proterozoic.